Variable cross-section plated mushroom with stud for bumping

ABSTRACT

An improved bump fabrication process is described that produces a larger diameter/taller solder ball than with a standard mushroom by forming an elongated mushroom having a short axis in the direction of adjacent connection mushrooms and an elongated axis orthogonal to the short axis. The increased larger volume solder when reflowed produces the larger diameter/taller bolder ball bump.

This Application claims priority under 35 USC 119(e)(1) of U.S.Provisional Application 60/346,394, filed Jan. 9, 2002.

The present application is a divisional of application Ser. No.10/339,377 filed Jan. 9, 2003 now U.S. Pat. No. 6,750,134.

FIELD OF INVENTION

This invention relates to semiconductor devices and in particular toputting solder balls on semiconductor chips for connection onto a wafer.

BACKGROUND OF INVENTION

As die sizes decrease with more components (e.g., circuits, transistors,or the like) to provide more functions there are more connection pointscloser together. The distance between the connection points or the pitchbecomes increasingly smaller as the die sizes decrease. Wire bonding ofthe connection points is used where these connection points are closetogether. It is highly desirable to provide solder bumps of larger andtaller size to enable these chips to be directly connected to asubstrate, package or circuit assembly without wire bonding. It ishighly desirable to provide a solder ball on the die with as large adiameter and as high a height as possible without bridging the othersolder balls to facilitate connections thereto. With a conventionalcircular resist defined, electroplated solder bump, the volume of platedsolder and the resulting reflowed solder bump height is limited by thepitch of the product. The pitch is the distance from the center of onebump to the center of an adjacent bump.

SUMMARY OF INVENTION

In accordance with one embodiment of the present invention a variablecross-section plated mushroom with stud will allow the bumping of alarger diameter, taller bump on a tight pitch product.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates side by side a die surface with photoresist wellaccording to the prior art on the right and photoresist well-doubleexposure according to one embodiment of the present invention on theleft.

FIG. 2 is an off-axis view showing side by side the conventional resistwell on the right and multi-cross sectioned resist well according to oneembodiment of the present invention on the left.

FIG. 3 illustrates side by side a conventional photoresist well withplated copper stud and solder mushroom on the right and a photoresistwell with plated copper stud and solder mushroom according to oneembodiment of the present invention on the left.

FIG. 4 illustrates side by side conventional plated copper studs andsolder mushrooms with photo resist stripped off on the right and soldermushrooms plated copper studs and solder mushrooms with photo resiststripped off according to one embodiment of the present invention on theleft.

FIG. 5 is a top view of die showing prior art plated solder mushrooms onthe upper right and resulting reflowed solder bumps on the lower rightand solder mushrooms according to one embodiment of the presentinvention on the upper left and resulting reflowed solder bumps on thelower left.

FIG. 6 is an off-axis view of die showing prior art plated soldermushrooms on the upper right and resulting reflowed solder bumps on thelower right and solder mushrooms according to one embodiment of thepresent invention on the upper left and resulting reflowed solder bumpson the lower left.

FIG. 7 is a flow chart of the process according to one embodiment of thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Referring to FIG. 1 there is illustrated a die 11 having on the topsurface a photoresist layer 13. On the right is a conventionalphotoresist well 15 for forming a conventional reflowed solder bump. Thephotoresist well 15 on the right is a conventional well using a singleexposure for a copper stud and plated solder mushroom with the copperstud and plated solder mushroom shaft having approximately the samediameter. On the left is a photoresist well 25 according to anembodiment of the present invention to produce a higher aspect ratioplated mushroom where the resulting reflowed bump is larger withoutshorting to the adjacent bump. The aspect ratio is higher with respectto the lateral direction toward the adjacent bump. It requires a doubleexposure with a first exposure through for the copper stud 26 and asecond exposure at blind depth for additional solder plating 27.

The conventional well 15 is mostly circular and is illustrated on theright by the two wells 15 a and 15 b in the off-axis view of FIG. 2. Thewell 25 according to one embodiment of the present invention has acircular portion 26 for a copper stud region and has a high aspect ratioshape portion 27 that is elliptical for the plating region 27 asillustrated by the two wells 25 a and 25 b. The short axis is in thedirection of the closest adjacent wells or solder mushrooms or betweenthe two wells 25 a and 25 b. The long axis is in the directionorthogonal to the short axis. The fabrication will use a stepper and amulti-field reticle to pattern thick resist to plate a variable crosssection mushroom. The process involves multiple layers ofphotolithography to create the variable cross-sectioned well. The firststep for the stud may be made by a first exposure set for a deep depthand set for a round focus for the stud region and then set for ashallower depth at an elliptical focus. There are many other ways toprovide the stepped well of round and then elliptical shape.

Referring to FIG. 3 there is shown at the right a conventional copperstud 31 and solder mushroom 33 electroplated in the well 15. When thesolder is plated, it fills the well forming the stem of the mushroom.Once the well is filled, the solder “overplates” onto the resist surfaceforming the cap of the mushroom. On the left side of FIG. 3 is shown thecopper stud 35 and the elongated electroplated solder mushroom 37 in thephotoresist well 25 according to one embodiment of the presentinvention. The dashed area 36 represents the additional solder enabledby the unique resist well design. Other variable cross-section shapesmay be used to increase the volume of electroplated solder mushroom suchas a fan shape or a diamond shape with the minor axis in the directionof the adjacent mushrooms.

FIG. 4 illustrates the identical copper studs 31 and 35 but differentsolder mushrooms 33 and 37 of FIG. 3 with the photoresist removed. Thesolder mushrooms 35 and 37 are ready for reflow.

FIG. 5 is a top view and FIG. 6 is an off axis view of plated soldermushrooms 41-43 of conventional type on the upper right and plated, highaspect ratio solder mushrooms 45-47 of the type according to anembodiment of the present invention on the upper left. The soldermushrooms are heated and reflowed to form the solder balls. Upon reflow,the solder will collapse into a spherical shape. On the lower right arethe reflow solder balls 51-53 resulting from heating the mushrooms41-43. Plating of the conventional circular mushroom in tight pitchproduct is limited by the pitch of the pads or the spacing between thecenters of the bond pads. On the lower left are the reflow solder balls55-57 resulting from heating the elongated mushrooms 45-47. Theadditional volume of solder enables by the elongated mushroom results ina spherical shape that is larger and taller than the conventional solderball even though the pitch is the same.

The size and direction of the ellipse permit the same spacing betweenthe centers of the solder balls. Other shapes are possible within thescope of the present invention. The direction of the elongated axis or aspur from the center is in a direction away the closest solder ball.

FIG. 7 illustrates the steps according to one embodiment of the presentinvention. The first step is providing a photoresist over a die. Thesecond step is forming multiple layers of photolithography to provide avariable cross-section well including a circular portion for a stud andan elliptical portion or other such noncircular shape in the directionaway from the adjacent wells. The third step is forming a copper stem inthe circular portion and electroplating the solder mushroom in theelliptical portion in the well of the photoresist. The fourth step isremoving the photoresist. The fifth step is reflowing the soldermushroom to form the solder ball.

The present invention involves modifying a standard bump fabricationprocess that involves photolithography and electroplating to produce animproved larger bump. The process allows for plating of tighter pitchwire bond or bump product. Plating of standard circular mushroom ontight pitch product is limited by the pitch of the target pads. Themajor diameter of the circular mushroom can not be any greater than thebump pitch. With the variable cross sectioned plated mushroom, thephotolithography well into which the mushroom is plated can be designedto have an aspect ratio greater than 1. That is the height to width inthe direction of adjacent balls is greater than 1. As an example, theelongated mushroom well can be designed so that the minor limitingdimension of the mushroom is the wire bond pad or bump pitch but themajor dimension is greater than this pitch thus allowing a greatervolume of solder to be plated. Upon reflow, the solder will collapseinto a spherical shape. The additional volume of solder enabled by theelongated mushroom will result in a larger/taller reflowed solder bump.

While the invention has been described in the context of preferredembodiments, it will be apparent to those skilled in the art that thepresent invention may be modified in other ways and may assume manyembodiments other than that specifically set out and described above.Accordingly various modifications can be made without departing from thescope of the invention.

1. An improved mushrooms for bumping of a semiconductor product having aseries closely space connections comprising: a stem extending from theclosely spaced connections on the semiconductor product; and anelongated mushroom extending from said stem; said elongated mushroomhaving a short axis in the direction of adjacent connection mushrooms tobe bumped and a long axis orthogonal to the short axis to increase thevolume of solder to be reflowed and produce a larger diameter/tallersolder ball than with a circular mushroom.
 2. The structure of claim 1wherein said stem is circular.
 3. The structure of claim 2 wherein saidcircular stem is copper.
 4. The structure of claim 1 wherein saidelongated mushroom is elliptical.